1. Field of the Invention
The invention relates to offshore drilling and production operations and is specifically directed to drilling and production tensioners and methods for installing and removing drilling and production tensioners.
2. Description of Related Art
A marine riser system is employed to provide a conduit from a floating vessel at the water surface to the blowout preventer stack or, production tree, which is connected to the wellhead at the sea floor. A tensioning system is utilized to maintain a variable tension to the riser string alleviating the potential for compression and in turn buckling or failure.
Historically, conventional riser tensioner systems have consisted of both single and dual cylinder assemblies with a fixed cable sheave at one end of the cylinder and a movable cable sheave attached to the rod end of the cylinder. The assembly is then mounted in a position on the vessel to allow convenient routing of wire rope which is connected to a point at the fixed end and strung over the movable sheaves. A hydro/pneumatic system consisting of high pressure air over hydraulic fluid applied to the cylinder forces the rod and in turn the rod end sheave to stroke out thereby tensioning the wire rope and in turn the riser.
Other prior tensioners require superstructure that restricts to the top or the bottom of the tensioner the location of workover equipment and other devices that are placed in-line with the drilling or production string. These conventional type tensioning systems have required high maintenance during normal operations due to the constant motion producing wear and degradation of the wire rope members. Replacing the active working sections of the wire rope by slipping and cutting raises safety concerns for personnel and has not proven cost effective. In addition, available space for installation and, the structure necessary to support the units including weight and loads imposed, particularly in deep water applications where the tension necessary requires additional tensioners poses difficult problems for system configurations for both new vessel designs and upgrading existing vessel designs.
Other problems unaddressed by these prior tensioners include the location of workover and other equipment placed in-line with the production or drill string and the high cost, labor, and increased safety concerns posed with the installation and removal of these tensioners.
The tensioner system of the present invention is an improvement over existing conventional and direct acting tensioning systems. Beyond the normal operational application to provide a means to apply variable tension to the riser, the system provides a number of enhancements and options including vessel configuration and its operational criteria. The tensioners of the invention may include additional equipment or components to facilitate quick and efficient installation and removal of the additional equipment along with the tensioner. For example, workover equipment such as workover units and snubbing units, may be disposed within the area formed by the tensioning cylinders. Additionally, a rotary table may be formed as part of the tensioner such that instead of removing the rotary table, inserting the tensioner, and replacing the rotary table, the tensioner can be installed with the rotary table already secured to the tensioner. Because of this novel design, the rotary table beams which support the rotary table may be moved to create a larger opening for larger tensioners. The larger opening also provides flexibility in the location of the tensioning cylinders to facilitate placement of other equipment, e.g., workover equipment.
Therefore, the tensioner and the methods of the present invention provide the advantages of: providing relatively quicker and safer installation of tensioners and other equipment; providing flexibility in location of equipment in fluid communication with the tensioners; eliminating offset and the resulting unequal loading in the event one or more of the tensioning cylinders fail; providing a system that is modular in construction, transportation, and assembly; providing interchangeability with other drilling or production facilities; reducing the amount of time that the wellhead is “idle,” i.e., that either a drilling riser or production riser is out of use by facilitating quick and easy installation of the tensioner, rotary table, and workover equipment; providing sufficient tension to the long string of the riser in deepwater over extended periods of time; providing a means to maintain the riser in constant tension, with, if necessary, overpull, while the riser is in service; providing the capability to accommodate angular offset between the riser and the vessel induced by vessel motion; and providing the capability to accommodate axial torque induced in the riser string in the event the drilling or production vessel rotates around the wellhead due to weather and sea conditions.